Ok, so this is an area where I've done ALOT of testing! I've tested every single LMP1, LMP2, GT1, group C car etc. in the entire game just for this event.
I actually did this hardcore style, I did it in one sitting, only paused for food and toilet! it took about 26 hours in total.
For B-spec you choice is'nt too hard, the car just needs to be fast enough for bob to win, and easy enough to drive for him, I recommend the Audi R8 playstation team oreca, but the C60 hybride, regular Audi R8, Sauber C9, Toyota minolta, Nissan R92CP will also do fine. YOU NEED TO FULLY TUNE THEM THO! bob generally sucks. you dont need the red bull, its too fast. you still need to do 24 hour, you just end up doing more laps.
For A-spec: your choice of car depends on many factors, but if you just want to get it over with with minimum effort, take the Red bull, stock setup is good, just do an oilchange. drive for about 18 hours and park the car at the finnishline the last 6 hours and do something fun meanwhile.
Now, if you want to do this properly, the main thing to consider is how YOU percieve the car. You'll be driving this car for 24 hours, or even 48 hours if you plan to do the nurburgring event aswell. this ofcourse differs from person to person, but to help you on the way there are a couple things to consider.
1. What level of difficulty do you want? (supertight all the way with the possibility to actually not win, kind of close but with no chance of loosing, or superior speed and easy victory?)
2. What camera angle are you planning to use? (cockpit view and on top of hood eliminates alot of candidates)
3. Are you going to use a wheel or a DS3? (how the cars behave differ a bit)
4. How is your driving style? (this is more of a tuning issiue, but ever so important)
5. Do you like how the car sounds? (like I said, you'll be driving this car for 48 hours, everything counts)
I'll use myself as an example. I wanted a kind of close race so that i dont get too bored, but i have to win, im not going to risk loosing the damn thing. So cars like the Panoz Esperante and Gillet Vertigo are out, so are the FGT and F1 cars.
Next, I use the "on top of hood" cam, and i want a camera angle that suits me, this eliminated the Audi R10, Sauber C9, 787B, nissan R89C etc due to the camera being too low down for my taste.
Next, all premiums was eliminated because of the hevier damage modelling. this is 24 hours, I will inevitable crash a few times no matter how good I am. And I dont like the car damaged, even if its only cosmetic. Hell even a dirty car annoys me!
Next the Bentley speed 8 was eliminated because i did'nt like the sound and the Nissan R390 was eliminated because i did'nt like how it felt in general.
this left me with 6 cars; the Audi R8's, the pescaloro GV5 and C60 hybride, the Toyota GT-one and Minolta.
theese where all good cars for the event, but I prefer a modern car over the group C minolta, they are easier to drive and they just suited me better. then i decided the Audis looked better than the Pescaloros and the steering wheel was on the right side. the Audis also felt a little bit better than the Toyota GT-one, in every way, just a tiny little, but still.
So, the two Audi R8's remain, and since the Playstation team oreca looks better and has an option to add extra downforce to make it even easier to drive, it was the car I chose.
There is no way im going to use a steering wheel for 24 hours, even if im faster with it, being faster is not the point here, general feel and driveability is. So what i ended up doing was raising the rear ride height, harder rear spring rate, harder LSD (25/50/10 is good i think) adding 100kg ballast and move it all the way to the front, stock engine and turbo with power limiter 90%, traction control to 7, no other aids, and Racing Hard all around.